1. Field of the Invention
The present invention relates to a semiconductor laser which can be electrically tuned over an oscillation wavelength range, and more particularly to a wavelength-tunable semiconductor laser of multi-electrode DFB (Distributed Feedback) structure in which each region in a DFB resonator can be independently controlled by a plurality of electrode.
2. Description of the Related Art
In recent years, researches and development have been increasingly made in optical frequency division-multiplexing (optical FDM). Optical FDM can be applied to various fields, such as large-capacity optical communication, optical inter-connection, optical switching, and optical arithmetic operation. A semiconductor laser is being developed as a coherent light source for use in optical FDM is of the type which is compact and reliable and which can be electrically tuned over an oscillation wavelength.
For large-capacity FDM it is required that a semiconductor laser be developed which can be turned over a broad oscillation wavelength range. To multiplex many frequency channels in high density within the predetermined wavelength range, it is desirable that the frequency range occupied by each channel be narrowed. In particular, a coherent-light transmission system, which is a feasible optical FDM application, needs a light source which emits a light beam having a small spectral linewidth to obtain an electric signal from interference between a signal light beam and a locally-oscillated light beam.
Also under development is an external-cavity semiconductor laser which has a wavelength-selecting device outside a semiconductor chip. The external-cavity semiconductor laser, however, is not recommendable for use in optical FDM in view of its size, stability, price, and reliability. A monolithic wavelength tunable semiconductor laser is required to achieve optical FDM. The wavelength-tunable semiconductor lasers, now under research, are classified into the follow four types:
(1) Temperature-control type semiconductor laser, PA1 (2) Multi-electrode distributed Bragg-reflection (DBR) semiconductor laser, PA1 (3) Multi-electrode distributed feedback (DFB) semiconductor laser, PA1 (4) Twin-guide semiconductor laser.
A temperature-control type semiconductor laser has a heating means located near the active region to raise the temperature of the active layer. Generally, the wavelength range of a semiconductor laser shifts to the long side as the temperature rises. This phenomenon is known as "red shift." The laser can therefore be turned over a broad wavelength range while a small linewidth is maintained. Since its active layer is heated considerably, however, the laser cannot be as reliable as is desired.
In a multi-electrode DBR semiconductor laser, carriers are injected into the Bragg reflection region, thereby changing the Bragg wavelength greatly. Certainly, the multi-electrode DBR laser can be tuned over a broad oscillation wavelength range. However, since carriers are injected into the Bragg wave guide which is passive, the internal carrier density varies. Consequently, this laser emits a beam having a linewidth as much as 10 MHz or more, and can hardly be used in a coherent-light transmission system. Further, the Bragg wavelength is changed, jumping over many modes, the multi-electrode DBR semiconductor laser can be merely continuously tuned but over a narrow oscillation wavelength range.
A multi-electrode DFB semiconductor laser has a several sections divided along a direction of a resonator. The balance of the carrier densities in those sections are changed, thereby varying the average carrier density. As a result, the oscillation wavelength of the laser is changed.
In the case of the 3-electrode phase-sifted DFB laser shown in FIG. 1 which has anti-reflection coatings on both faces, when a large current is uniformly injected into the laser, as indicated in FIG. 2A by the solid curve, light will concentrate near the phase-shifting section as shown in FIG. 2B by the solid curve. Induced emission at the phase-shifting section therefore increases, whereby the carrier density is lower at the center portion of the device than at the ends thereof, as is indicated in FIG. 2C by the solid curve. Due to the non-uniform carrier density (i.e., axial hole-burning), the average carrier density is higher than in the case where the current density is uniform along the axis of the device. The higher the average carrier density, the lower the refractive index, and the shorter the oscillation wavelength.
When more current is injected into the center portion of the laser than to the end portions thereof, as indicated in FIG. 2A by the broken curve, the carrier density is substantially uniform as is indicated in FIG. 2C by the broken curve. The average carrier density therefore decreases, whereby the device undergoes a red shift, or its wavelength range shifts to the long side. Since the entire region is active, its average carrier density does not varies so much at the time of current injection. A small linewidth can be therefore realized. However, since the average carrier density does not change so much, the changeable wavelength range is limited to about 2 nm only.
In a twin-guide semiconductor laser, the currents supplied to the active layer and the light-guiding layer (or the second active layer), which are closely laminated, are controlled independently. This device is, as it were, divided into layers in the direction of the thickness, as compared to the multi-electrode DBR or DFB laser which is divided into several sections in the axial direction. The device operates in a mode similar to the operating mode of a multi-electrode DBR laser if the waveguide layer, which is a passive element, is used, and in a mode similar to the operating mode of a multi-electrode DFB laser if the waveguide layer is replaced by a second active layer.
In both the multi-electrode DBR laser and the multi-electrode DFB laser, not only the above-mentioned carrier effect but also the heat effect much contributes to the changes in oscillation wavelength. In the laser shown in FIG. 1 and FIGS. 2A to 2C, too, the temperature rise caused by the current increase in the center portion greatly contributes to the red shift.
The multi-electrode DBR laser and the twin-guide semiconductor laser can hardly be applied to a coherent-light transmission system, since these lasers emit a beam whose spectrum linewidth is is not sufficiently small. Nor can the temperature-control type semiconductor laser be recommended for use in a coherent-light transmission system, since the laser is not so reliable as desired due to the heat applied to the active layer to change the oscillation wavelength. Therefore, the multi-electrode DFB semiconductor laser seems suitable for use in a coherent-light transmission system.
Technique called "gain-levering" is known as a method of enhancing the wavelength tunability of a multi-electrode DFB semiconductor laser. The principle of gain-levering is discussed in K. Y. Lau, Appl. Phys. Lett., Vol. 57(25), pp. 2632-2634 (December 1990). This technique will be summarized as follows, with reference to FIGS. 3 and 4.
FIG. 3 shows a gain-levered DFB quantum well semiconductor laser. FIG. 4 is a graph illustrating the relationship between carrier density and gain, which is observed with this gain-levered DFB quantum well laser. As is evident from FIG. 4, the laser has a high non-linearity since its gain saturates in the region having a high carrier density.
It is assumed that the operating points of the regions 1 and 2 of the laser are located at point A (uniformly excited condition) on the curve shown in FIG. 4. As the current injected into the region 1 is decreased, gradually lowering the carrier density in the region 1, the carrier density in the region 2 increases to maintain the gain at the initial value. As a result, the operating points of the regions 1 and 2 shift to points B1 and B2 as is illustrated in FIG. 4. Due to the non-linearity of the curve representing the relationship between the carrier density and the gain, the average carrier density of the laser increases as the operating points of the regions 1 and 2 shift from the point A (uniform excitation) to the points B1 and B2 (non-uniform excitation). The oscillation wavelength therefore shifts toward the short side. (That is, a blue shift takes places.) Due to the change in the current, the gain and the carrier density (i.e., the oscillation wavelength) greatly change in the regions 1 and 2, respectively. In view of this, the region 1 is considered as a gain-control region, and the region 2, as a wavelength control region.
If the excitation is rendered non-uniform by virtue of gain-lever effect, thus effecting a blue shift, however, not only the average carrier density but also the average current density will increase, inevitably raising the temperature of the laser. This temperature rise causes a red shift, which cancels out the blue shift. Consequently, the gain-levered DFB quantum well laser cannot be tuned over so broad a wavelength range as is expected.
In the case of a multi-electrode DFB semiconductor laser, the refractive-index distribution in the resonator varies with the carrier-density distribution in the resonator. Hence, the wavelength at which the laser requires a minimum threshold gain to oscillate does change. On the other hand, the change in the refractive-index distribution in the resonance alters the phase-matching condition. If the wavelength at which the laser requires a minimum threshold gain to oscillate always satisfies the phase-matching condition, it can be expected that the laser continues stable oscillating.
With any conventional multi-electrode DFB laser, however, it is difficult to simultaneously control the threshold-gain condition and the phase-matching condition. The conventional multi-electrode DFB laser oscillates stably but over a small oscillation wavelength range. In a three-electrode phase-shifted DFB laser, for example, when the current injected into the center section of the resonator is increased, while maintaining the currents injected into the end regions at a constant value, the refractive-index distribution along the length of the resonator varies, inevitably changing both the threshold-gain condition and the phase-matching condition. Such a mode jump as shown in FIG. 19 is therefore unavoidable. As a consequence, the wavelength range over which the laser can be tuned in one oscillation mode is limited to 1 nm or less.
The conventional multi-electrode DFB laser has another problem. As a current is injected into each region of the laser, not only the carrier density but also the temperature distributes non-uniformly. Due to the non-uniform carrier density distribution and the non-uniform temperature distribution, the refractive index, which depends on both the carrier density and the temperature, is not distributed uniformly, either. Because of these complex oscillation conditions, it is more difficult to control the operating mode of the multi-electrode DFB laser.